Pathogenicity of Aeromonas veronii causing mass mortalities of Odontobutis potamophila and its induced host immune response

Silica nanoparticles alleviate the immunosuppression, oxidative stress, biochemical, behavioral, and histopathological alterations induced by Aeromonas veronii infection in African catfish (Clarias gariepinus)

In the aquaculture industry, silica nanoparticles (SiNPs) have great significance, mainly for confronting diseases. Therefore, the present study aims to assess the antibacterial efficiency of SiNPs as a versatile trial against Aeromonas veronii infection in African catfish (Clarias gariepinus). Further, we investigated the influence of SiNPs in palliating the immune-antioxidant stress biochemical, ethological, and histopathological alterations induced by A. veronii. The experiment was conducted for 10 days, and about 120 fish were distributed into four groups at random, with 30 fish each. The first group is a control that was neither exposed to infection nor SiNPs. The second group (SiNPs) was vulnerable to SiNPs at a concentration of 20 mg/L in water. The third group was experimentally infected with A. veronii at a concentration of 1.5 × 107 CFU/mL. The fourth group (A. veronii + SiNPs) was exposed to SiNPs and infected with A. veronii. Results outlined that A. veronii infection induced behavioral alterations and suppression of immune-antioxidant responses that appeared as a clear decline in protein profile indices, complement 3, lysozyme activity, glutathione peroxidase, and total antioxidant capacity. The kidney and liver function biomarkers (creatinine, urea, alkaline phosphatase, and alanine aminotransferase) and lipid peroxide (malondialdehyde) were substantially increased in the A. veronii group, with marked histopathological changes and immunohistochemical alterations in these tissues. Interestingly, the exposure to SiNPs resulted in a clear improvement in all measured biomarkers and a noticeable regeneration of the histopathological changes. Overall, it will establish that SiNPs are a new, successful tool for opposing immunological, antioxidant, physiological, and histopathological alterations induced by A. veronii infection.

Comparison of the efficacy of Aeromonas veronii ΔhisJ vaccine in Carassius auratus via different immunization routes

Introdction

Aeromonas veronii is a significant pathogen to various aquatic life. Infections in fish can lead to high mortality rates, causing substantial economic losses in aquaculture. Vaccination is proposed as a substitute for antibiotics in aquaculture to decrease disease-related mortality and morbidity. Our study previously constructed a hisJ-deleted strain of A. veronii, which provided protective effect to Loach.

Methods

To further assess the vaccine's applicability, this study evaluated its genetic stability and safety, and the immune protective effects in Carassius auratus through four distinct administration routes: intraperitoneal injection, intramuscular injection, oral administration, and immersion, to determine the efficacy of these administration routes.

Results

The results showed that the vaccine remained genetically stable after 45 generations. Immunization via these administration routes was safe for Carassius auratus, with intraperitoneal and intramuscular injections causing stronger adverse reactions. Immersion immunization resulted in mild adverse reactions, and no significant adverse reactions were observed following oral immunization. Immunizing Carassius auratus at safe concentrations via these routes enhanced the phagocytic activity in serum, increased the levels of non-specific immune-related enzymes (ACP, AKP, C3, C4, LZM, SOD, and IgM), and improved specific serum antibody levels. It also elevated levels of cytokines related to inflammatory responses (IL-1β, IL-10, TNF-α, TGF-β) in organ tissues (liver, spleen, kidney, mid-post intestine, and gills). The survival rates of Carassius auratus were measured after challenging with the virulent strain A. veronii TH0426, resulting in the relative survival rates of 64% for Intraperitoneal vaccine group, 56% for Intramuscular vaccine group, 52% for oral vaccine group, and 48% for immersion vaccine group. Analysis of bacterial load in the liver, spleen, and kidney post-challenge showed a decreasing trend in the control group, indicating that the vaccine strain ΔhisJ could gradually restrict the rapid proliferation of bacteria in these tissues, thereby providing a certain level of immune protection against A. veronii.

Discussion

In brief, the vaccine strain ΔhisJ can serve as a safe live attenuated vaccine for Carassius auratus, and this study lays the foundation for the development of live attenuated vaccines against Aeromonas veronii.

Characteristics and Complete Genome Analysis of a Pathogenic Aeromonas Veronii SJ4 from Diseased Siniperca Chuatsi

As an opportunistic pathogen, Aeromonas veronii can cause hemorrhagic septicemia of various aquatic animals. In our present study, a dominant strain SJ4, isolated from naturally infected mandarin fish (Siniperca chuatsi), was identified as A. veronii according to the morphological, physiological, and biochemical features, as well as molecular identification. Intraperitoneal injection of A. veronii SJ4 into S. chuatsi revealed clinical signs similar to the natural infection, and the median lethal dosage (LD50) of the SJ4 to S. chuatsi in a week was 3.8 × 105 CFU/mL. Histopathological analysis revealed that the isolate SJ4 could cause cell enlargement, obvious hemorrhage, and inflammatory responses in S. chuatsi. Detection of virulence genes showed the isolate SJ4 carried act, fim, flgM, ompA, lip, hly, aer, and eprCAL, and the isolate SJ4 also produce caseinase, dnase, gelatinase, and hemolysin. In addition, the complete genome of A. veronii SJ4 was sequenced, and the size of the genome of A. veronii SJ4 was 4,562,694 bp, within a G + C content of 58.95%, containing 4079 coding genes. Nine hundred ten genes encoding for several virulence factors, such as type III and VI secretion systems, flagella, motility, etc., were determined based on the VFDB database. Besides, 148 antibiotic resistance-related genes in 27 categories related to tetracyclines, fluoroquinolones, aminoglycosides, macrolides, chloramphenicol, and cephalosporins were also annotated. The present results suggested that A. veronii was etiological agent causing the bacterial septicemia of S. chuatsi in this time, as well as provided a valuable base for revealing pathogenesis and resistance mechanism of A. veronii.

First Report of Aeromonas veronii as an Emerging Bacterial Pathogen of Farmed Nile Tilapia (Oreochromis niloticus) in Brazil

Brazil is one of the world's leading producers of Nile tilapia, Oreochromis niloticus. However, the industry faces a major challenge in terms of infectious diseases, as at least five new pathogens have been formally described in the last five years. Aeromonas species are Gram-negative anaerobic bacteria that are often described as fish pathogens causing Motile Aeromonas Septicemia (MAS). In late December 2022, an epidemic outbreak was reported in farmed Nile tilapia in the state of São Paulo, Brazil, characterized by clinical signs and gross pathology suggestive of MAS. The objective of this study was to isolate, identify, and characterize in vitro and in vivo the causative agent of this epidemic outbreak. The bacterial isolates were identified as Aeromonas veronii based on the homology of 16S rRNA (99.9%), gyrB (98.9%), and the rpoB gene (99.1%). A. veronii showed susceptibility only to florfenicol, while it was resistant to the other three antimicrobials tested, oxytetracycline, enrofloxacin, and amoxicillin. The lowest florfenicol concentration capable of inhibiting bacterial growth was ≤0.5 µg/mL. The phenotypic resistance of the A. veronii isolate observed for quinolones and tetracycline was genetically confirmed by the presence of the qnrS2 (colE plasmid) and tetA antibiotic-resistant genes, respectively. A. veronii isolate was highly pathogenic in juvenile Nile tilapia tested in vivo, showing a mortality rate ranging from 3 to 100% in the lowest (1.2 × 104) and highest (1.2 × 108) bacterial dose groups, respectively. To our knowledge, this study would constitute the first report of highly pathogenic and multidrug-resistant A. veronii associated with outbreaks and high mortality rates in tilapia farmed in commercial net cages in Brazil.

Therapeutic efficacy of coriander (Coriandrum sativum) enriched diets in Oreochromis niloticus: effect on hepatic-renal functions, the antioxidant-immune response and resistance to Aeromonas veronii

In this study, the effects of Coriandrum sativum to control Aeromonas veronii infection in Oreochromis niloticus were determined. Coriandrum sativum extract (CE) was tested in vitro against A. veronii by the disc diffusion assay. In in vivo, 150 O. niloticus (from El-Abbassa, Sharkia, Egypt, weighing 34.95 ± 1.98 g) was distributed in five groups (with three replications) in glass aquariums (80 × 40 × 30 cm). The first group (control) was intraperitoneally injected with 0.2 ml of sterilized tryptic soya broth. Groups 2-5 were intraperitoneally challenged with 0.2 ml of A. veronii (4.3 × 106). The five groups were administered a basal diet until clinical signs appeared, and then therapeutic feeding (15 days) was followed: the first (CONT) and second (AV) groups were administered a normal basal diet. The third (AV+CP) and fourth (AV+CE) groups were administered diets supplemented with C. sativum powder and extract, respectively, each at 30 mg/kg. The fifth group (AV+OT) was administered a diet supplemented with oxytetracycline at 500 mg/kg diet. The results of the in vitro experiment revealed that CE has a zone of inhibition of 43 mm against A. veronii. The in vivo results showed that fish administered a therapeutic diet supplemented with CE showed a significant improvement in hematological, biochemical, and immunological parameters, as well as antioxidant capacity (P < 0.05) and the pathological findings of the liver and kidney tissues. The current findings supported that the administration of a CE-enriched diet (30 mg/kg) is an eco-friendly strategy for controlling A. veronii in O. niloticus.

Immunological effects of recombinant Lactobacillus casei expressing pilin MshB fused with cholera toxin B subunit adjuvant as an oral vaccine against Aeromonas veronii infection in crucian carp

Aeromonas veronii is a zoonotic agent capable of infecting fish and mammals, including humans, posing a serious threat to the development of aquaculture and public health safety. Currently, few effective vaccines are available through convenient routes against A. veronii infection. Herein, we developed vaccine candidates by inserting MSH type VI pili B (MshB) from A. veronii as an antigen and cholera toxin B subunit (CTB) as a molecular adjuvant into Lactobacillus casei and evaluated their immunological effect as vaccines in a crucian carp (Carassius auratus) model. The results suggested that recombinant L. casei Lc-pPG-MshB and Lc-pPG-MshB-CTB can be stably inherited for more than 50 generations. Oral administration of recombinant L. casei vaccine candidates stimulated the production of high levels of serum-specific immunoglobulin M (IgM) and increased the activity of acid phosphatase (ACP), alkaline phosphatase (AKP) superoxide dismutase (SOD), lysozyme (LZM), complement 3 (C3) and C4 in crucian carp (carassius auratus) compared to the control group (Lc-pPG612 group and PBS group) without significant changes. Moreover, the expression levels of interleukin-10 (IL-10), interleukin-1β (IL-1β), tumour necrosis factor-α (TNF-α) and transforming growth factor-β (TGF-β) genes in the gills, liver, spleen, kidney and gut of crucian carp orally immunized with recombinant L. casei were significantly upregulated compared to the control groups, indicating that recombinant L. casei induced a significant cellular immune response. In addition, viable recombinant L. casei can be detected and stably colonized in the intestine tract of crucian carp. Particularly, crucian carp immunized orally with Lc-pPG-MshB and Lc-pPG-MshB-CTB exhibited higher survival rates (48% for Lc-pPG-MshB and 60% for Lc-pPG-MshB-CTB) and significantly reduced loads of A. veronii in the major immune organs after A. veronii challenge. Our findings indicated that both recombinant L. casei strains provide favorable immune protection, with Lc-pPG-MshB-CTB in particular being more effective and promising as an ideal candidate for oral vaccination.

The Dynamics of Gene Expression Unraveling the Immune Response of Macrobrachium rosenbergii Infected by Aeromonas veronii

To further investigate the immune response of Macrobrachium rosenbergii against Aeromonas veronii, comparative transcriptomic analyses of the M. rosenbergii hepatopancreas were conducted on challenge and control groups at 6, 12, and 24 h post-infection (hpi), independently. A total of 51,707 high-quality unigenes were collected from the RNA-seq data, and 8060 differentially expressed genes (DEGs) were discovered through paired comparisons. Among the three comparison groups, a KEGG pathway enrichment analysis showed that 173 immune-related DEGs were considerably clustered into 28 immune-related pathways, including the lysosome, the phagosome, etc. Moreover, the expression levels of the four key immune-related genes (TOLL, PAK1, GSK3β, and IKKα) were evaluated at various stages following post-infection in the hepatopancreas, hemolymph, and gills. Both PAK1 and GSK3β genes were highly up-regulated in all three tissues at 6 hpi with A. veronii; TOLL was up-regulated in the hepatopancreas and hemolymph but down-regulated in the gill at 6 hpi, and IKKα was up-regulated in hemolymph and gill, but down-regulated in the hepatopancreas at 6 hpi. These findings lay the groundwork for understanding the immune mechanism of M. rosenbergii after contracting A. veronii.

Single-cell transcriptome analysis reveals a cellular immune response in freshwater dark sleeper (Odontobutis potamophila) after infection with Aeromonas veronii

The bacterium Aeromonas veronii is a co-pathogenic species that can negatively impact the health of both humans and aquatic animals. In this study, we used single-cell transcriptome analysis (scRNA-seq) to investigate the effects of infection with A. veronii on head kidney cells and the regulation of gene expression in the dark sleeper (Odontobutis potamophila). scRNA-seq was used to assess the effects of infection with A. veronii in O. potamophila B cells, endothelial cells, macrophages, and granulocytes, and differential enrichment analysis of gene expression in B cells and granulocytes was performed. The analyses revealed a significant increase in neutrophils and decrease in eosinophils in granulocytes infected with A. veronii. Activation of neutrophils enhanced ribosome biogenesis by up-regulating the expression of RPS12 and RPL12 to fight against invading pathogens. Crucial pro-inflammatory mediators IL1B, IGHV1-4, and the major histocompatibility class II genes MHC2A and MHC2DAB, which are involved in virulence processes, were upregulated, suggesting that A. veronii activates an immune response that presents antigens and activates immunoglobulin receptors in B cells. These cellular immune responses triggered by infection with A. veronii enriched the available scRNA-seq data for teleosts, and these results are important for understanding the evolution of cellular immune defense and functional differentiation of head kidney cells.

Hematological, immuno-antioxidant disruptions, and genes down-regulation induced by Aeromonas veronii challenge in Clarias gariepinus: The ameliorative role of silica nanoparticles

Aeromonas veronii is a pathogenic bacterium associated with various diseases in aquaculture. However, few studies address the antibacterial activity using nanoparticles (NPs). Hence, the current study is innovative to evaluate the antibacterial efficacy of silica nanoparticles (SiNPs) against A. veronii infection in-vitro with a trial for treatment in-vivo. Primarily, we assessed the in-vitro antibacterial activity against A. veronii. Further, we investigated the hematological profile, immune-antioxidant response, and gene expression of African catfish (Clarias gariepinus) in response to SiNPs exposure and the A. veronii challenge. Fish (N = 120; weight: 90 ± 6.19 g) were distributed into four groups (30 fish/group) for a ten-days-treatment trial. The first (control) and second (SiNPs) groups were treated with 0 mg/L and 20 mg/L SiNPs in water, respectively. The third (A. veronii) and fourth (SiNPs + A. veronii) groups were treated with 0 mg/L and 20 mg/L SiNPs in water, respectively, and infected with A. veronii (1.5×10⁷ CFU/mL). Results demonstrated that SiNPs displayed an in-vitro antibacterial activity against A. veronii with a 21 mm inhibitory zone. A. veronii infection caused a high mortality rate (56.67%) and substantial reductions in hematological indices and immune indicators [nitric oxide (NO) and immunoglobulin M (IgM)]. Additionally, marked decline in the level of antioxidants [superoxide dismutase (SOD), catalase (CAT), and reduced glutathione content (GSH)] as well as down-regulation in the immune-related genes [interleukins (IL-1β and IL-8) and tumor necrosis factor-alpha (TNF-α)] and antioxidant-related genes [SOD1, glutathione peroxidase (GPx), and glutathione-S-transferase (GST)] were the consequences of A. veronii infection. Surprisingly, treatment of A. veronii-infected fish with SiNPs lessened the mortality rate, enhanced the blood picture, modulated the immune-antioxidant parameters, and resulted in gene up-regulation. Overall, this study encompasses the significant role of SiNPs, a new versatile tool for combating hematological, immuno-antioxidant alterations, and gene down-regulation induced by A. veronii infection and sustainable aquaculture production.

Isolation, Identification, and Characterization of Aeromonas veronii from Chinese Soft-Shelled Turtle (Trionyx sinensis)

Aeromonas veronii is widespread in aquatic environments and is capable of infecting various aquatic organisms. A. veronii infection is lethal for Chinese soft-shelled turtles (Trionyx sinensis, CSST). We isolated a gram-negative bacterium from the liver of diseased CSSTs, which was named XC-1908. This isolate was identified as A. veronii based on its morphological and biochemical characteristics, and 16S rRNA gene sequence analysis. A. veronii was pathogenic for CSSTs with an LD₅₀ of 4.17 × 10⁵ CFU/g. The symptoms of CSSTs artificially infected with isolate XC-1908 were consistent with those of the naturally infected CSSTs. The levels of total protein, albumin, and white globule in the serum samples of the diseased turtles were decreased, whereas those of aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase were elevated. Moreover, the diseased CSSTs exhibited the following histopathological changes: the liver contained numerous melanomacrophage centers, renal glomerulus were edematous, intestinal villi were shed, and in oocytes, the number of vacuoles increased and red-rounded particles were observed. Antibiotic sensitivity tests revealed that the bacterium was sensitive to ceftriaxone, doxycycline, florfenicol, cefradine, and gentamicin, and resistant to sulfanilamide, carbenicillin, benzathine, clindamycin, erythromycin, and streptomycin. This study provides control strategies to prevent outbreaks of A. veronii infection in CSSTs.

Construction and efficacy of Aeromonas veronii mutant Δhcp as a live attenuated vaccine for the largemouth bass (Micropterus salmoides)

Aeromonas veronii is a human and animal co-pathogenic bacterium that could have a significant negative impact on both human health and aquaculture. In this study, a mutant strain of A. veronii with deletion of the hemolysin co-regulated protein (hcp) gene was constructed (Δhcp-AV). Compared with the wild strain, Δhcp-AV showed significantly reduced growth capacity and biofilm formation ability. Motility tests showed that the hcp gene had no significant effect on the swimming and swarming ability. In addition, the pathogenicity was also reduced. To evaluate the efficacy of Δhcp-AV as a live attenuated vaccine for prevention of Aeromonas veronii infection, we compared the immune response of largemouth bass (Micropterus salmoides) after immunization with 500 μL of 1.47 × 105 CFU/mL of Δhcp-AV and 4 × 108 CFU/mL of inactivated A. veronii. Obvious increases of serum immune related enzyme activity were observed in immunization groups. Expression levels of immune-related genes in Δhcp-AV group were up-regulated, and higher than those in inactivated A. veronii group. After challenging with live A. veronii, the relative percent survival (RPS) was 100% in Δhcp- AV group, whereas the RPS was 76.67% in inactivated A. veronii group. Our data suggest that the live attenuated vaccine Δhcp- AV could elicit a stronger immune response and provide a higher RPS than inactivated A. veronii. These data suggest that hcp gene is an important virulence factor of A. veronii, and the live attenuated vaccine Δhcp-AV is safe and effective for prevention A. veronii infection in M. salmoides farming.

Aeromonas veronii Is a Lethal Pathogen Isolated from Gut of Infected Labeo rohita: Molecular Insight to Understand the Bacterial Virulence and Its Induced Host Immunity

A case of severe mortality in farmed Labeo rohita was investigated to characterize the causative agent. We identified the bacterial strain as Aeromonas veronii isolated from the gut of infected L. rohita by biochemical assay, scanning electron microscopy and 16S rRNA gene sequence analysis. The in vivo challenge experiment showed that the LD₅₀ of A. veronii was 2.2 × 10⁴ CFU/fish. Virulence gene investigation revealed that the isolated A. veronii possesses Aerolysin, Cytotoxic enterotoxin, Serine protease, Dnase and Type III secretion system genes. The isolated strain was resistant to two antibiotics (ampicillin and dicloxacillin) while susceptible to 22 other antibiotics. The study further revealed that A. veronii induced both stresses along with non-specific and specific immune responses marked by elevated cortisol HSP70, HSP90 and IgM levels in the treated L. rohita fingerlings. Although the bacterial pathogen enhances the immune response, the negative effect on fish, including stress, and high mortality, create concern and a need for A. veronii management in L. rohita farms. The knowledge gained from this study would facilitate future research aimed at assessing the pathogenicity of A. veronii, with an emphasis on microbial disease management in other farmed fish species.

Pathogenicity of Aeromonas veronii Causing Mass Mortality of Largemouth Bass (Micropterus salmoides) and Its Induced Host Immune Response

Aeromonas veronii is as an important opportunist pathogen of many aquatic animals, which is wildly distributed in various aquatic environments. In this study, a dominant bacterium GJL1 isolated from diseased M. salmoides was identified as A. veronii according to the morphological, physiological, and biochemical characteristics, as well as molecular identification. Detection of the virulence genes showed the isolate GJL1 carried outer membrane protein A (ompA), flagellin (flgA, flgM, flgN), aerolysin (aer), cytolytic enterotoxin (act), DNases (exu), and hemolysin (hly), and the isolate GJL1 also produced caseinase, lipase, gelatinase, and hemolysin. The virulence of strain GJL1 was confirmed by experimental infection; the median lethal dosage (LD50) of the GJL1 for largemouth bass was 3.6 × 105 CFU/mL, and histopathological analysis revealed that the isolate could cause obvious inflammatory responses in M. salmoides. Additionally, the immune-related gene expression in M. salmoides was evaluated, and the results showed that IgM, HIF-1α, Hep-1, IL-15, TGF-β1, and Cas-3 were significantly upregulated after A. veronii infection. Our results indicated that A. veronii was an etiological agent causing the mass mortality of M. salmoides, which contributes to understanding the immune response of M. salmoides against A. veronii infection.